This invention relates to a recording system for use in coding a sequence of image signals into a sequence of coded image signals and in recording the coded image signal sequence on a main image recording medium.
Attempts have been made to provide a reproduction system which uses a read-only memory in the form of a compact disk (abbreviated to a CD-ROM hereinafter) as an image source for education or amusement purposes. For convenience of reference, the CD-ROM will be referred to below as "a main image recording medium".
According to CCIR Recommendation 601 which defines a specification for CD-ROMS, CD-ROMS should have a maximum readout rate of 150 kilobytes and a memory capacity of 540 megabytes. As long as the CD-ROM complies with this specification, i.e., standard, a single frame of image signals should be coded into a coded signal of 5 kilobytes, with the condition that thirty frames of image signals appear for one second. It is difficult, if not impossible, to condense such a frame into 5 kilobytes of data. Therefore, efficient coding or redundancy reduction coding is indispensable for recording or reproducing the image signals. Otherwise, it is impossible to reproduce a sequence of moving images from the CD-ROM. Efficient coding will be more simply referred to herein as information reduction.
If information reduction is possible such that a single frame of the image signals is coded into a coded signal of 5 kilobytes, a lengthy reproduction of about would be storable on a CD-ROM.
Recently, a proposal has been offered as regards a color image recording CD-ROM which realizes a reproduction of about one hour, as a usual audio signal recording CD-ROM. In this case, an image signal is recorded on the color image recording CD-ROM in the form of a predictive error signal appearing as a result of efficient coding carried out in a manner to be described later. Such efficient coding may be, for example, interframe coding. When the interframe coding is used to code an image including a still portion and a moving portion having a small area in comparison with the still portion, it is possible to reduce the amount of significant information at the still portion.
However, use of such interframe coding renders an area of the moving portion undesirably small as compared with an actual area thereof on a reproduction of the image. When the image includes the moving portion which is quickly moving, loss of scene information might take place in the interframe coding. This results in a reduction of both frame rate and spacial resolution. Such a reduction of a frame rate gives rise to a degradation of temporal resolution and therefore to blurring of a reproduced image.